1. Field of the Invention
The present invention relates to a voltage superposing circuit in use with an electronic tuner, and more particularly to a voltage superposing circuit for superposing a tuning voltage supplied to an electronic tuner used in a liquid crystal television receiver, for example, on a voltage for automatic fine tuning.
2. Description of the Related Art
An electronic tuner used for a liquid crystal television (TV) receiver, for example, is generally composed of a tuning circuit, a local oscillator, and a mixer. In the tuning circuit, a resonance frequency is varied in accordance with the frequency of a TV channel selected. To vary the resonance frequency, a channel select circuit applies a tuning voltage to the tuning circuit, more exactly, a bias voltage to a variable diode contained in the tuning circuit. An oscillating frequency of the local oscillator varies depending on ambient temperature, aging of components contained therein. To maintain a correct local oscillating frequency, an automatic fine tuning (AFT) circuit is provided, which produces an AFT voltage for correcting a deviation of the oscillating frequency due to such factors, and applies it to the electronic tuner.
The AFT circuit detects a deviation from a center frequency of an intermediate frequency (IF) derived from the electronic tuner, and produces a DC voltage (AFT voltage) in proportion to the detected deviation. The AFT voltage controls the local oscillating frequency of the tuner so that the deviation of the IF center frequency f0 is removed. The result is stabilization of the IF frequency.
FIG. 1 shows a relationship between an AFT voltage vs. a shift of the center frequency f0 of the IF frequency. In the graph, an ordinate represents the AFT voltage and an abscissa represents the shift of the center frequency f0. V0 indicates a reference voltage providing the center frequency f0 (58.75 MHz in the Japanese TV standard). As seen from the graph, when the center frequency of an IF signal outputted from the electronic tuner is shifted from the center frequency f0 to the higher (+) side, the AFT circuit produces an AFT voltage lower than the voltage V0. On the other hand, when it is shifted to the lower (-) side, the AFT circuit produces an AFT voltage higher than the voltage V0. In this way, a predetermined pull-in range to stabilize the IF frequency is set up by the AFT voltage.
The electronic tuner thus constructed may be classified into two types. One of those types of tuner is provided with two input terminals, one receiving an AFT voltage and the other for receiving a tuning voltage. The other type of the tuners is provided with only one input terminal for receiving a tuning voltage. The latter type of the tuner requires a circuit to control a tuning voltage directly by an AFT voltage, viz, a voltage superposing circuit for superposing an AFT voltage on the tuning voltage.
An example of the voltage superposing circuit follows. A channel select voltage terminal receives a channel select voltage. This voltage is generated when a viewer manually operates a channel select knob of the TV set for selecting his desired channel. The terminal is connected to the base of a first npn transistor. The transistor is connected at the collector to a first voltage source terminal. The emitter of the transistor is connected through a first resistor to a tuning voltage terminal of the electronic tuner. An AFT voltage terminal receives an AFT voltage from an AFT circuit. The AFT voltage terminal is connected to a connection point between second and third resistors. These resistors are connected in series between a second voltage source terminal and a reference potential point. The connection point between the second and third resistors is connected through a fourth resistor to the emitter of a second npn transistor. The second transistor is connected at the collector to the tuning voltage terminal of the electronic tuner. The base of the second transistor is connected to a connection point between fifth and sixth resistors. These fifth and sixth resistors are connected in series between the second voltage source and the reference potential point. The emitter of the second transistor is connected through a seventh resistor to the reference potential point.
In the voltage superposing circuit thus constructed, a voltage appearing the connection point between the second and third resistors corresponds to the voltage V0. The voltage depends on a resistance ratio of those resistors. A current flowing from the first resistor to the collector of the second transistor varies in accordance with the AFT voltage applied to the AFT voltage terminal. A variation of the tuning voltage applied to the tuning voltage terminal of the tuner provides a voltage component to automatically control a local oscillating frequency of the tuner. Assuming that resistance of the first resistor is R, and a current flowing from the first resistor to the collector of the second resistor is I, a variation of the current I is .DELTA.I and a variation of the tuning voltage, i.e., an AFT voltage component, is R.times..DELTA.I.
Another example of the voltage superposing circuit will be given below. A channel voltage select terminal for receiving a channel select voltage is connected through an eighth resistor to a tuning voltage terminal of an electronic tuner. An AFT terminal is connected to a connection point between ninth and tenth resistors which are connected in series between a third voltage source terminal and the reference potential point. The connection point is connected through an eleventh resistor to the tuning voltage terminal of the tuner.
Assuming that a resistance of the eighth resistor is R' and a current flowing from the eighth resistor to the eleventh resistor is I' when the voltage, V0, appears at the connection point between the ninth and tenth resistors, an AFT voltage component is given by R'.times..DELTA.I'.
Relationships between the tuning voltage and the frequencies of TV channels are illustrated in FIG. 2. In the figure, the ordinate indicates a tuning voltage VT, and the abscissa indicates channel number. A curve A represents a variation of the tuning voltage VT in the VHF low band; a curve B represents a variation of the tuning voltage VT in the VHF high band; and a curve C represents a variation of the tuning voltage VT in the UHF band.
It is noted that these curves indicating the tuning voltage vs. channel number characteristics are non-linear, and that a frequency variation corresponding to the variation of the tuning voltage VT becomes greater, the higher the channel number is. In the tuner operating in the AFT mode, the pull-in range in the lower channel numbers is broad, while that in the higher channel numbers is narrow, because the varied voltage component R.times..DELTA.I, for example, is constant over the entire range of the channel numbers. The nonuniform AFT pull-in ranges make the receiving characteristics of the TV set nonuniform over the range of the TV channels. This indicates that some channels are received in satisfactory conditions, but some channels are received in unsatisfactory conditions.